Clock signals are essential elements in large-scale digital integrated circuits. In these digital systematic circuits, normally there are three kinds of clock signal generation circuits: RC (resistance-capacitance) oscillators, ring oscillators and crystal oscillators. Ring oscillators have a wide range of oscillation frequency and relatively high stability, but have less resistance to power supply noises and may occupy relatively large layout areas. Crystal oscillators have an accurate oscillation frequency and may work stably since the precision thereof may only be related to the intrinsic frequency of selected crystal components. However, crystal oscillators have drawbacks like larger power consumption, higher price. Besides, it is difficult for a crystal oscillator to be integrated in a chip. RC oscillators, due to their characteristics like simple structure, lower power consumption, low cost, convenience for being integrated, and the like, are widely used in oscillation circuits of integrated circuits.
However, output frequency of RC oscillators may be greatly influenced by work voltages and environmental temperatures. Besides, characteristics of RC oscillators may be highly related to manufacturing processes. For example, due to the limitations of semiconductor manufacturing, oscillators formed at different positions of a same wafer may have disparity in oscillation frequencies. Therefore, conventional oscillators may need to be calibrated to output more precise frequencies.